1. Field of the Invention
The present invention relates to a clad aluminum alloy material having a good brazability and high strength and corrosion resistance after brazing and suitable for use as a tube material, a header plate material, etc., which are structural members used in the production of an Al heat exchanger, such as a radiator or a heater core, by brazing in an inert gas atmosphere by using of a fluoride flux or brazing in vacuum, particularly suitable for the fabrication of tubes having a small wall thickness.
2. Description of the Prior Art
A clad material having a three-layer structure, comprising a core material consisting of an Al-Mn-base alloy, such as 3003, clad on one surface with an Al-Si-base brazing filler metal and on the other surface with a sacrificial anode material of an Al-Zn-base alloy or Al-Zn-Mg-base alloy, has been used as tube materials or header plate materials for automobile radiators, heater cores, etc. Throughout the specification, aluminum alloy numbers represent the designations by JIS (Japanese Industrial Standards). The Al-Si-base brazing filler metal is used for joining a tube to a fin and joining a tube to a header plate. In many cases, the brazing is conducted in an inert gas atmosphere by using of a fluoride flux or vacuum brazing. The other surface of the core material clad with the sacrificial anode material is located inside (on the side of water) during use to exhibit a sacrificial anode action which prevents the occurrence of pitting corrosion of the core material or crevice corrosion.
In recent years, there is an ever-increasing demand for a reduction in the weight of radiators, heater cores, etc., which requires a reduction in the wall thickness of the tube materials or header plate materials. An increase in the strength, particularly an improvement in the strength after brazing, is necessary for this purpose. This has led to a tendency that Mg is added to the core material for the purpose of increasing the strength. However, Mg lowers the corrosion resistance and, at the same time, is detrimental to the brazability. Specifically, in the case of brazing using a fluoride flux, Mg diffuses into the surface during brazing and reacts with the fluoride flux, so that a flocculent product (a fluoride of Mg) is formed, which brings about deposition or joining failure. In the vacuum brazing as well, Mg is detrimental to brazing. For this reason, the amount of addition of Mg to the core material is limited to 0.5% at the maximum, practically 0.2 to 0.3%, which is obstacle to an increase in the strength.
There is a possibility that the strength of the tube material or header plate material can be increased also by adding Mg to the sacrificial anode material. Several proposals have hitherto been made on clad materials wherein Mg was added to the sacrificial anode material.
Specifically, the following proposals have been made on the addition of Mg to the sacrificial anode material of header materials or tube materials for radiators:
(1) a method wherein Mg and Zn or the like are incorporated (see Japanese Patent Publication No. 28704/1988);
(2) Zn and Mg are added (see Japanese Patent Laid-Open No. 89498/1986);
(3) Sn and Mg are simultaneously added (see Japanese Patent Laid-Open Nos. 16646/1981 and 89641/1988);
(4) Mg and Zn are added in relatively large amounts (see Japanese Patent Publication No. 45301/1987); and
(5) Mg or a combination of Mg with Zn or the like is added (see Japanese Patent Laid-Open No. 175093/1990).
In the methods (1) and (2), Mg is added in an amount as small as 1.1% or 1.5% or less for the purpose of preventing pitting corrosion or crevice corrosion. In these methods, no improvement in the strength is attained.
In the method (3), Mg is added for the purpose of inhibiting the intergrannular diffusion of Sn to prevent the occurrence of cracking during hot rolling. In the method (4), Mg is added for the purpose of improving the pitting corrosion resistance. In all the methods, there is a possibility that, when the Mg content is high, Mg diffuses to attain the effect of improving the strength to some extent. In the method (5), the strength is improved through the diffusion of Mg into the core material. When a thin-walled tube material (a clad material) is prepared, although the strength of the core material can be increased by virtue of Mg diffusion from the sacrificial anode material, mere addition of Mg alone causes the strength of the sacrificial anode material to be unsatisfactory, so that the strength of the clad material as a whole cannot be increased. That is, in a reduction in the wall thickness, the contribution of not only the core material but also the sacrificial anode material to the strength becomes large, which makes it necessary to increase the strength of the sacrificial anode material as well.
Accordingly, an object of the present invention is to provide a clad material which can attain a high strength after brazing with the amount of addition of Mg to the core material being limited to 0.5% at the maximum and prevents the thickness of the sacrificial anode layer from becoming excessively large.
The present inventors have made studies on a method to ensure a high strength even after brazing while limiting the amount of addition of Mg to the core material to 0.5% at the maximum and, as a result, have found that an addition of large amounts of Mg and Si to a sacrificial anode material causes a part of the Mg in the sacrificial anode material to diffuse into the core material during brazing and, thereby, to strengthen the core material, that the sacrificial anode material per se is strengthened by Mg and Si, and that although an increase in the Si content of the sacrificial anode material brings about intergranular corrosion when the cooling rate after brazing is small, the intergranular corrosion can be prevented by appropriately regulating the Si content to a suitable value, which has led to the completion of the present invention.
Specifically, Mg is allowed to exist in the sacrificial anode material together with Si in such an amount as will not be detrimental to the corrosion resistance in order for the Mg to contribute to the strengthening of the core material and, at the same time, for the purpose of strengthening the sacrificial anode material through solid solution strengthening by Mg and Si and age hardening strengthening by the precipitation of Mg.sub.2 Si.